The present invention relates to an improved countercurrent solid-liquid contacting apparatus, and more particularly to column crystallizer apparatus employing the use of crystal beds to suppress undesirable axial liquid mixing thereby increasing separation efficiency.
Column crystallizer apparatus are generally used for purifying organic mixtures by countercurrently contacting solute crystals and mother liquor. In a typical column crystallizer, moving internals such as screws, scrapers, trays or balls are employed to transport crystal solids from one end of the column to the other end where the crystals are melted. A portion of the crystal melt is withdrawn as product while the rest is refluxed. The column internals can also be used to promote mass transfer between crystal solids and mother liquor thereby decreasing the size of equilibrium stages.
For example, the column crystallizer disclosed in British Pat. No. 1,365,536 comprises a plurality of vibrating perforated plates positioned at spaced intervals in the column. A plurality of freely movable bodies are disposed on the surface of the perforated plates. The entire column assembly is subjected to vertical reciprocations so that the movable bodies agitate and bounce on the perforated plates. The action of the movable bodies comminute any crystal aggregates formed. A scraped surface cooling unit, located at the top of the column, produces the initial crop of crystals. A heating unit, located at the bottom, melts the purified crystals thereby permitting easy product recovery.
The effectiveness of the above-described apparatus in obtaining high purity crystals is limited due to axial liquid mixing in the column. Experimental evidence indicates that liquid on each side of a vibrating plate mixes due to flow through the plate perforations and the clearance between the plate circumference and the column wall. Axial liquid mixing causes impure mother liquor in the top of the column to travel downward and mix with purer crystal product melt in the bottom of the column, thereby greatly reducing the separation efficiency of the column. Axial liquid mixing can be reduced by operating with a more concentrated crystal slurry. However, operating at high crystal slurry concentrations inhibits the comminuting action of the movable bodies.
Axial liquid mixing is known to become more severe in large diameter columns. The designer of commercial scale crystallization equipment must also contend with the problems of liquid channeling and by-passing found in large diameter columns. Because of these difficult mixing problems, only limited success has been realized on scale-up of pilot plant column crystallizers to commercial size equipment.
It is the overall object of the present invention to provide an improved column crystallizer apparatus.
It is, therefore, an object of the present invention to provide a set of improved internals for reducing the axial liquid mixing in a column crystallizer apparatus.
It is another object of the present invention to provide an improved column crystallizer apparatus capable of efficiently crystallizing systems having small differences between solid and liquid densities.
It is still another object of the present invention to provide an efficient column crystallizer apparatus which can be economically scaled-up to commercial size.
These and other objects and advantages will become apparent to those skilled in the art from the following description and figures.